The term optics is used in reference to the appearance or perception of something.
The senator knew that he was perfectly capable of accepting the contribution from Colonel Randolph and supporting the bill because it was the right thing to do and not because of pressure from a big donor. However, everyone knew that the Colonel had a reputation for trying to buy influence and the optics of his having taken the money and coincidently voting for the bill would make him look corrupt even if it was merely a coincidence. He decided that he did not want to take the risk of taking the colonel’s money. It just wasn’t worth the risk of looking like he had been bought off.